This is a continuation grant for a study concerned with patients with intact A-V conduction and intraventricular conduction defects (bundle branch block). The project was initiated on NIH ccntract 71-2478 under the sudden death program. Patients with bundle branch block are evaluated clinically, electrocardiographically, and electrophysiologically (including recording of His bundle potentials, measurement of pacing responses and refractory periods, and evaluation of sinus node function). Patients are then followed prospectively in conduction disease clinics with specific goals of detecting significant bradyarrhythmia and sudden death. The hypothesis being tested is that electrophysiological studies will allow delineation of high risk groups (as determined by subsequent follow-up). All initial and follow-up data is computerized, allowing analysis with rapid information retrieval, multi-variate analysis, and life table methods. BIBLIOGRAPHIC REFERENCES: Dhingra R, Amat-y-Leon F, Wyndham C, Deedwania PC, Wu D, Denes P, Rosen KM: Clinical significance of prolonged sino-atrial conduction time. Circulation 55: 8, 1977. Bharati S, Dhingra RC, Lev M, Towne WD, Rahimtoola SH, Rosen KM: Conduction system studies in a case of prinzmetal's angina with transient A-V block. Amer J Card, 39: 120, 1977.